In order to evaluate the performance of rotating machines or parts thereof, it is desirable to represent the vibrational characteristics in terms of magnitude and phase versus frequency, otherwise known as a power spectrum. In order to produce the power spectrum of vibration-based data, uniform time interval data must be processed to convert the time-based vibration data into frequency-based data. This process is generally effective when the rotational speed is constant; though not accurate when the rotational speed is variable.
One well-known approach for obtaining and converting time-based vibration data from a fixed speed shaft to frequency-based data is to sample the time-based vibration data synchronously with the sensing of the rotational speed of the shaft. The sampling done at a fixed shaft speed provides signals useful in obtaining magnitude and phase measurements at a once-per-revolution (1/REV) frequency. However, when the shaft speed is changing during the interval for which the vibration signal is being sampled, the magnitude and phase measurements for the sampled data will be inaccurate.
The prior art utilizes three different methods for obtaining vibration measurements for variable speed shafts, namely analog tracking filters, switched capacitor tracking filters, and digital tracking filters, each of which is used in conjunction with a tachometer, for sensing speed and generating a shaft speed signal, and a vibration sensor, for sensing acceleration, velocity or displacement.
The analog tracking filter multiplies the sense vibration signal by the fundamental, 1/REV, shaft speed signal. Sum and difference frequencies result in accordance with well-known principles. The difference frequency is extracted with a low-pass filter, the output of which is further processed to produce a voltage proportional to the vibration amplitude. The low-pass filter must have a wide bandwidth to account for measurement inaccuracies due to the variability of the speed of the shaft. Therefore, the analog tracking filter method suffers from poor noise rejection due to the use of a wider than optimal pass band.
The switched capacitor tracking filter uses discrete time sampling techniques to synthesize stable high-accuracy multi-pole filters, with the cut-off frequencies being controlled by a clock. The speed sensor signal is used to generate the switched capacitor filter clock. The output of the filter is then further processed to produce voltages proportional to vibration amplitude and phase angle. The switched capacitor tracking filter technique includes filtering delays for rejecting tachometer jitter. Moreover, when the speed sweeps dynamically, mistracking of the filter results, due to time delays in responding to the speed variations. To compensate for the poor dynamic performance, the tracking filer bandwidth is increased, resulting, as with the analog tracking filter, in less noise rejection.
Finally, digital tracking filters are utilized which use digital processing techniques to implement the tracking filter through software. Digital filtering of the tachometer permits measurement by the tachometer on a 1/REV speed and removal of tachometer jitter by averaging of the tachometer frequency on a 1/REV basis. With sufficient processing capability, the cut-off frequencies of the tracking filter may be tracked to the latest speed measurement. One approach, the subject of co-pending application Ser. No. 946,913, filed simultaneously herewith, entitled "Dynamic Digital Tracking Filter" of Garcia, et al. and assigned to the present assignee, uses a Fast Fourier Transform (FFT) to estimate the vibration spectrum while associated software defines the appropriate band of frequencies based upon the speed for the measurement period. A further enhancement, order tracking, uses an integer multiple of the 1/REV sampling rate to center the 1/REV vibration signal within the tracking filter to thereby eliminate amplitude variations within the filter.
Digital tracking filters suffer from some of the same deficiencies as the two previously-discussed filters. To ensure tracking, the digital tracking filter bandwidths must be wide enough to maintain the fundamental rotor speed within the bands. Consequently, the bandwidths include energy outside of the fundamental rotor speed, thereby increasing the errors associated with unwanted vibrations. Order tracking, described above, can narrow the bandwidth, but is still susceptible to inaccuracies due to speed variations and non-synchronous tones. In all cases, the delay between measuring the speed and estimating the frequency spectrum introduces mistracking errors. Further, differences in the processing of the speed and vibration signals can introduce system errors which affect the output of the filter.
It is therefore an objective of the present invention to provide a tracking filter which overcomes the deficiencies of prior art tracking filters.
It is another objective of the invention to obtain accurate measurement of the absolute magnitude and the relative phase of vibration of a rotating structure having uniform or non-uniform rotating speed.
Yet another objective of the present invention is to produce optimum absolute magnitude and relative phase of vibration measurements for a given number of rotations of the structure or for a specific time window.
A further objective of the invention is to produce optimum relative phase and absolute magnitude measurements for a given rotational speed variation.
A still further objective is to dynamically provide the minimum tracking bandwidth for the actual speed variation.
Another objective of the invention is to provide a tracking filter which eliminates the effects of delay in estimating the frequency spectrum and further eliminates the effects of noise and of system-introduced errors.
A further objective of the invention is to provide a tracking filter which eliminates the need to perform phase correction due to the position of the 1/REV frequency within the tracking band.